1. Field of the Invention
The invention relates to drill strings having a wireline extending down therethrough from surface readout means to a downhole probe such as a survey or steering tool employed to sense azimuth and drift angle during directional drilling.
2. Description of the Prior Art
Directional drilling is widely utilized today for the efficient exploitation of oil fields, but requires great accuracy in drill orientation as to both azimuth and inclination or drift angle.
Straight sections of the bore are drilled with a simple rotary drill, and required alterations in the direction of the bore are achieved using a downhole mud motor drill of either the positive displacement or turbine type which is offset at a small angle relative to the drill string axis by means of a bent sub. The bent sub causes the drill to follow a path which curves away from the straight section of the bore above it, and steering is accomplished by rotation of the drill pipe at the surface to the desired azimuthal orientation of the bent section. The amount of deflection or inclination from the straight section of bore above is determined by the length of the curved path that is followed by the drill at the desired azimuth.
A requirement of directional drilling is that the azimuth and inclination angles of the drill be monitored at the surface. Originally, the only type of instruments available for such purpose were pivoted and liquid damped compasses which aligned themselves with the earth's magnetic and gravitational fields. The readings of such downhole instruments had to be recorded by means of a photograph, and such instruments were not adaptable to remote reading because of the difficulties of incorporating an electrical transducer without introducing an error-producing torque onto the delicate pivoted elements. Thus, each reading of the instrument required retrieval of the measuring instrument at the surface which was time-consuming and expensive, and even then only static tool orientations could be read. At considerable depths, as for example depths in excess of about 10,000 feet, it is extremely difficult to accurately steer the drill because reaction torque twists the drill pipe to some indeterminate wind-up angle, and therefore accurate direction drilling at depth requires continuous surface monitoring of the drill orientation.
A downhole survey or steering tool capable of providing a continuous surface readout through a wireline data transmitting conductor was developed in the late 1960's by Russell Attitude Systems of Cheltenham, England, and several survey or steering tools of this type are currently being marketed in the United States, as for example by Sperry-Sun and by Eastman Whipstock, both of Houston, Tex.
While such survey or steering tools have solved the problem of constant monitoring of both azimuth and inclination angle at the surface during directional drilling, they still have the disadvantage that each time a stand of drill pipe is added to the drill string during directional drilling, the survey or steering tool and its wireline must be tripped up out of the drill string before the additional stand of drill pipe is added to the string, and then the survey or steering tool and wireline must be tripped back down through the drill string and the survey or steering tool reengaged in its properly oriented, operative position associated with the drill.
Such trips of the survey or steering tool and wireline are costly and time-consuming. Thus, wireline companies currently charge from 20.cent. to 35.cent. per foot of bore depth for each round trip of the wireline, plus hourly rates for the truck, an operator, and a helper. Tripping of the wireline out of and back into the drill string at a depth of about 5,000 feet requires approximately 21/2 to 3 hours. An additional expense is a current charge of $500 per day by each of the companies which now provide such survey or steering tools and associated surface readout equipment, this charge including two operators plus tools. A still further charge is made by the directional drilling company, such companies currently charging an average of $400 per day for operator plus tools. Thus, in addition to the direct per foot trip cost made by the wireline company, each time the survey or steering tool and wireline are tripped there are large additional costs incurred to the wireline company, the company providing the survey or steering tool and associated surface readout equipment, and the directional drilling company, in proportion to the required trip time.